In recent years, HBTs comprising GaAs and related compounds (hereinafter referred to as GaAs series HBTs) have been developed for high-power output microwave devices. However, since the GaAs series HBTs have high thermal resistance, the junction temperature undesirably increases when used as high-power output devices. This undesired increase in the junction temperature is suppressed in a prior art HBT structure described by Burhan Bayraktaroglu et al in IEEE Electron Device Letters, Vol. 14 (1996), pp. 493-495. FIG. 26 shows a cross-sectional view of the prior art HBT. In the figure, reference numeral 26 designates a GaAs substrate. A plurality of HBT elements, i.e., unit elements, are arranged in an array on the surface of the GaAs substrate 26 and electrically connected in parallel with each other. Each HBT element includes an emitter electrode 21, a collector electrode 22, and a pair of base electrodes 23. Metal wirings 50 are disposed on the surface of the GaAs substrate 26 at opposite sides of the array of the HBT elements. The emitter electrodes 21 of the HBT elements are connected to an air-bridge wiring 24 having opposite ends connected to the metal wirings 50, and heat generated in the HBT elements are transferred through the air-bridge wiring 24 and the metal wirings 50 to the GaAs substrate 26. Thereby, more than the quantity of heat dissipated through the substrate directly under the HBT elements is dissipated.
However, in the prior art structure shown in FIG. 26, because the heat spreading regions through the air-bridge wiring 24 to the substrate 26 are present only on both sides of the array of the HBT elements, this structure still has the following drawbacks.
(1) Since the length of the air-bridge wiring 24 from each HBT element to the heat spreading region is long, the thermal resistance is not sufficiently reduced. PA1 (2) Since the length of the air-bridge wiring 24 from each HBT element to the heat spreading region is not uniform, the thermal resistance varies from element to element. PA1 (3) Since the length of the air-bridge wiring 24 from each HBT element to the heat spreading area is long, the emitter inductance is increased. PA1 (4) The air-bridge wiring 24 must be thicker than 10 .mu.m to improve the heat conduction of the air-bridge wiring 24, but it is difficult to fabricate such a thick air-bridge wiring. PA1 (5) Since the thermal separation between the HBT elements is not sufficient, the temperature of the elements in the center of the array unfavorably increases.
As described above, in the prior art HBT shown in FIG. 26, the heat dissipation property is improved to some extent by the air-bridge structure. However, because the air-bridge wiring is long, the improvement in the heat dissipation property is not sufficient. In addition, since the thermal resistance varies from element to element, the junction temperature varies from element to element, resulting in variations in the electrical characteristics of the HBT elements. Further, the large emitter inductance causes a reduction in gain when the device is used in a high frequency band. Furthermore, since the air-bridge wiring must be thick, the fabricating process is complicated. In addition, since the thermal separation between the HBT elements is insufficient, thermal interference occurs between the HBT elements, whereby the temperature of the elements in the center of the array unfavorably increases.